The invention relates to fiber optic ribbon type cable.
Excess fiber attenuation in optical fiber ribbon stacks has been a continuing problem to cablers. Optical fibers in ribbon form are made by disposing a plurality of optical fibers, with each optical fiber normally having its own individual plastic coating, in planar relationship one to another and then circumscribing the planar array with a common plastic matrix coating. A plurality of ribbons are arranged in a stack and a stack is then placed in a compartment such as a core tube, a groove in a slotted core type of structure, or a "U" shaped member stranded about a central member. When fiber optic cables containing stacks of ribbons were bent, installed/rewound and/or thermally cycled, it was found that the attenuation of certain fibers increased, especially the comer fibers. By comer fibers in the stack it is meant the four comers of the ribbon stack when viewed in cross section.
From the above observation it was concluded that the optical fiber ribbon plastic matrix to plastic matrix coefficient of friction was critical in order to enable the ribbons to slide easily, relative to one another. It is toward the solution to the problem of insufficient sliding that the invention is directed.
While Zeidler, U.S. Pat. No. 5,224,191, teaches the addition of powders or filling compounds to enhance ribbon sliding, it is believed that reducing ribbon friction without external sliding agents is needed, as cables not having filling compounds are desirable in some applications, such as for indoor cables.
Coating materials having a low modulus of elasticity generally have the disadvantage of having a high coefficient of friction.
European Patent Application 194,891 discloses an optical ribbon in which the same material, DESOLITE 950-075, serves as the coating on each of the individual optical fibers and also serves as the common bonding coating. Such material is described to be an ultraviolet-curable epoxyacrylate having a low tensile modulus. Such material is specified to have a secant modulus of 2.2 MPa at 2.5% strain according to product literature as published by its manufacturer.
Japanese laid-open publication 62-89915, published in 1987, discloses a slotted core optical cable having ribbon stacks comprising individual optical fiber ribbons each having an outer common coating having a static friction coefficient of 0.9 or less, such as Teflon or nylon, applied to the exterior of an inner common coating formed of ultraviolet curable resin. One such outer common coating is slated to have a static coefficient of friction of 0.65. However, making such a cable involves an additional coating step and the manufacturer must be assured of proper adhesion and material compatibility between the inner and outer common coating layers.